linux-kernel - Re: [RFC PATCH net-next v2 0/5] netns: allow to identify peer netns

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Message-ID: <CALCETrWLhfQpT+f=DPTsB4w5Oy+OeiygBrQosf25HDJQaOnPWg@mail.gmail.com>
Date:	Thu, 2 Oct 2014 12:48:14 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Network Development <netdev@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [RFC PATCH net-next v2 0/5] netns: allow to identify peer netns

On Thu, Oct 2, 2014 at 12:45 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
>> On Thu, Oct 2, 2014 at 12:20 PM, Eric W. Biederman
>> <ebiederm@...ssion.com> wrote:
>>> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
>>>
>>>> Le 29/09/2014 20:43, Eric W. Biederman a écrit :
>>>>> Nicolas Dichtel <nicolas.dichtel@...nd.com> writes:
>>>>>
>>>>>> Le 26/09/2014 20:57, Eric W. Biederman a écrit :
>>>>>>> Andy Lutomirski <luto@...capital.net> writes:
>>>>>>>
>>>>>>>> On Fri, Sep 26, 2014 at 11:10 AM, Eric W. Biederman
>>>>>>>> <ebiederm@...ssion.com> wrote:
>>>>>>>>> I see two ways to go with this.
>>>>>>>>>
>>>>>>>>> - A per network namespace table to that you can store ids for ``peer''
>>>>>>>>>     network namespaces.  The table would need to be populated manually by
>>>>>>>>>     the likes of ip netns add.
>>>>>>>>>
>>>>>>>>>     That flips the order of assignment and makes this idea solid.
>>>>>> I have a preference for this solution, because it allows to have a full
>>>>>> broadcast messages. When you have a lot of network interfaces (> 10k),
>>>>>> it saves a lot of time to avoid another request to get all informations.
>>>>>
>>>>> My practical question is how often does it happen that we care?
>>>> In fact, I don't think that scenarii with a lot of netns have a full mesh of
>>>> x-netns interfaces. It will be more one "link" netns with the physical
>>>> interface and all other with one interface with the link part in this "link"
>>>> netns. Hence, only one nsid is needing in each netns.
>>>
>>> I will buy that a full mesh is unlikely.
>>>
>>> For people doing simulations anything physical has a limited number of
>>> links.
>>>
>>> For people wanting all to all connectivity setting up an internal
>>> macvlan (or the equivalent) is likely much simpler and more efficient
>>> that a full mesh.
>>>
>>> So the question in my mind is how do we create these identifiers at need
>>> (when we create the cross network namespace links) instead of at network
>>> namespace creation time.  I don't see an answer to that in your patches,
>>> and perhaps it obvious.
>>>
>>
>> I wonder whether part of the problem is that we're thinking about
>> scoping wrong.  What if we made the hierarchy more explicit?
>>
>> For example, we could give each netns an admin-assigned identifier
>> (e.g. a 64-bit number, maybe required to be unique, maybe not)
>> relative to its containing userns.  Then we could come up with a way
>> to identify user namespaces (i.e. inode number relative to containing
>> user ns, if that's well-defined).
>
> If as suggested we only assign ids when a tunnel (or equivalent) is
> created between two network namespaces the space cost is a non-issue.
> The ids become at worst a constant factor addition to the cost of the
> tunnel.
>
> To keep things simple we may want to assign a free id (if one does not
> exist) when we connect a tunnel to a network namespace.
>
>> From user code's perspective, netnses that are in the requester's
>> userns or its descendents are identified by a path through a (possibly
>> zero-length) sequence of userns ids followed by a netns id.  Netnses
>> outside the requester's userns hierarchy cannot be named at all.
>>
>> Would this make sense?
>
> Nope.  What happens if I migrate 2 of the 4 network namespaces in a user
> namespace?  The migration potentially fails.  Application migration does
> not require user namespace migration.

Hmm.  I guess that, as long as those network namespaces aren't
connected to anything else, migrating like that makes sense and ought
to work.  Fair enough.

--Andy
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